1. Field of the Invention
The present invention relates to a method of fabricating an integrated circuit, and more particularly to a method of LPCVD (Low Pressure Chemical Vapor Deposition) silicon nitride deposition for decreasing particles.
2. Description of the Prior Art
Silicon nitride (Si.sub.3 N.sub.4) is a general dielectric material used in a semiconductor manufacturing process. Its primary application is to be a mask for silicon oxide (SiO.sub.2) etching. The silicon nitride mask property prevent oxidation of active regions on the substrate surface by its with respect to the of difficult oxygen penetration. The above description is process of LOCOS (Local Oxidation of Silicon). Besides the above application, silicon nitride has been provided to be a passivation of a semiconductor device by its property of difficult water penetration and alkali metal ions prevention. Because of having two different applications, silicon nitride is deposited by LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition) according to its required characteristics.
An LPCVD reactor has an operating pressure less than 1 torr (0.2.about.0.5 torr is the best). Thereby a film deposited by LPCVD will have a good ability of step coverage.
FIG. 1 shows a Tubular Type LPCVD reactor, which is a hot wall reactor, built from annealed quartz 2. A set of heater elements 4 called Five Zone Heater is located around the quartz tube 2. The reactants are input from the tube bottom inlet 6 into inner tube. The silicon wafer in a quartz boat is positioned to a proper position for deposition. And the exhaust gases are output from the exit 10 by a pumping system.
In the process of silicon nitride deposition through low pressure method, a silicon nitride film is usually deposited in a tube with high temperature for increasing deposition rate. After the deposition step, the tube is still under low pressure. Therefore, before removing the silicon wafer with a silicon nitride film from the tube, the tube has to be input clean inert gas (Ar, N.sub.2) from bottom inlets 6 and 8 for purging step and cycle purging step to remove dangerous gases inside the tube. And then the inner tube pressure is balanced with the environment pressure for removing silicon wafer easily. Typically, the input gas usually has a high gas flow rate of more than 1 L/min for decreasing the time of the cycle purging step. Thereby the process time inside the tube is decreased to improve efficiency. However, the high flow rate gas for cycle purging step has a lower temperature which will suddenly cool the hot wall of the tube. As a result, the silicon nitride film near the wall will delaminate to induce silicon nitride particles which would be blown to the surface of the silicon wafer. After a new reclean tube runs a process for 15 times, the amount of particles described above will rapidly increase to 100 (for the particle size about 0.2 microns), which is a big problem of product yield. A method of wet brush has been applied for decreasing particles, but a stress between the silicon nitride film and the substrate under it will be induced to make a bad effect in the following steps of exposure and development. Therefore, the conventional method of wet brush is not an efficient solution for the mentioned problem.
In order to solve the above problem and improve product yield, the present invention discloses a method of LPCVD silicon nitride deposition for decreasing particles by decreasing the gas flow rate in the cycle purging step. Thereby the silicon wafer surface contamination problem induced by conventional process can be avoided.